This invention relates to fiber optic waveguides. One advantage of a singe mode optical waveguide, and a restricted mode guide, over a multimode guide is the elimination or reduction of mode dispersion. A disadvantage is that, for a given core to cladding refractive index difference, the core diameter is smaller than that of a corresponding multimode fiber. A small core presents particular problems in the making of permanent and demountable butt joints between fibers because a lateral displacement of the axis of one fiber with respect to that of the other gives rise to a coupling loss that increases with a reduction in core size. The core diameter can be increased so as to reduce this misalignment problem, but this requires a reduction in the core to cladding refractive index difference. One result is that the mode or modes are less tightly bound to the core, and hence radiative losses at bends in the fiber are increased.